ESA’s Rosetta spacecraft has made the first measurement of molecular
nitrogen at a comet, providing clues about the temperature environment
in which Comet 67P/Churyumov–Gerasimenko formed.

Rosetta arrived last August, and has since been collecting extensive
data on the comet and its environment with its suite of 11 science
instruments.

The in situ detection of molecular nitrogenhas long
been sought at a comet. Nitrogen had only previously been detected bound
up in other compounds, including hydrogen cyanide and ammonia, for
example.

Its detection is particularly important since molecular nitrogen is
thought to have been the most common type of nitrogen available when the
Solar System was forming. In the colder outer regions, it likely
provided the main source of nitrogen that was incorporated into the gas
planets. It also dominates the dense atmosphere of Saturn’s moon, Titan,
and is present in the atmospheres and surface ices on Pluto and
Neptune’s moon Triton.

It is in these cold outer reaches of our Solar System in which the
family of comets that includes Rosetta’s comet is believed to have
formed.

The new results are based on 138 measurements collected by the Rosetta
Orbiter Spectrometer for Ion and Neutral Analysis instrument, ROSINA,
during 17–23 October 2014, when Rosetta was about 10 km from the centre
of the comet.

“Identifying molecular nitrogen places important constraints on the
conditions in which the comet formed, because it requires very low
temperatures to become trapped in ice,” says Martin Rubin of the
University of Bern, lead author of the paper presenting the results
published today in the journal Science.

The trapping of molecular nitrogen in ice in the protosolar nebula is
thought to take place at temperatures similar to those required to trap
carbon monoxide. So in order to put constraints on comet formation
models, the scientists compared the ratio of molecular nitrogen to
carbon monoxide measured at the comet to that of the protosolar nebula,
as calculated from the measured nitrogen to carbon ratio in Jupiter and
the solar wind.

That ratio for Comet 67P/Churyumov–Gerasimenko turns out to be about 25
times less than that of the expected protosolar value. The scientists
think that this depletion may be a consequence of the ice forming at
very low temperatures in the protosolar nebula.

One scenario involves temperatures of between roughly –250ºC and perhaps
–220ºC, with relatively inefficient trapping of molecular nitrogen in
either amorphous water ice or cage-like water ice known as a clathrate,
in both cases yielding a low ratio directly.

Alternatively, the molecular nitrogen could have been trapped more
efficiently at even lower temperatures of around –253ºC in the same
region as Pluto and Triton, resulting in relatively nitrogen-rich ices
as seen on them.

Subsequent heating of the comet through the decay of radioactive
nuclides, or as Rosetta’s comet moved into orbits closer to the Sun,
could have been sufficient to trigger outgassing of the nitrogen and
thus a reduction of the ratio over time.

“This very low-temperature process is similar to how we think Pluto and
Triton have developed their nitrogen-rich ice and is consistent with the
comet originating from the Kuiper Belt,” says Martin.

The only other body in the Solar System with a nitrogen-dominated
atmosphere is Earth. The current best guess at its origin is via plate
tectonics, with volcanoes releasing nitrogen locked in silicate rocks in
the mantle.

However, the question remains as to the role played by comets in delivering this important ingredient.

“Just as we wanted to learn more about the role of comets in bringing water to Earth,
we would also like to place constraints on the delivery of other
ingredients, especially those that are needed for the building blocks of
life, like nitrogen,” says Kathrin Altwegg, also at the University of
Bern, and principal investigator for ROSINA.

To assess the possible contribution of comets like Rosetta’s to the
nitrogen in Earth’s atmosphere, the scientists assumed that the isotopic
ratio of 14N to 15N in the comet is the same as that measured for Jupiter and solar wind, which reflects the composition of the protosolar nebula.

However, this isotopic ratio is much higher than measured for other
nitrogen-bearing species present in comets, such as hydrogen cyanide and
ammonia.

Earth’s 14N/15N ratio lies roughly between these
two values, and thus if there was an equal mix of the molecular form on
the one hand, and in hydrogen cyanide and ammonia on the other in
comets, it would be at least conceivable that Earth’s nitrogen could
have come from comets.

“However, the amount ofnitrogen found in
67P/Churyumov–Gerasimenko is not an equal mix between molecular nitrogen
and the other nitrogen-bearing molecules. Rather, there is 15 times too
little molecular nitrogen, and therefore Earth’s 14N/15N ratio cannot be reproduced through delivery of Jupiter family comets like Rosetta’s,” says Martin.

“It’s another piece of the puzzle in terms of the role of Jupiter family
comets in the evolution of the Solar System, but the puzzle is by no
means finished yet,” says ESA’s Rosetta project scientist, Matt Taylor.

“Rosetta is about five months away from perihelion now, and we’ll be
watching how the composition of the gases changes over this period, and
trying to decipher what that tells us about the past life of this
comet.”

Notes for Editors

“Molecular nitrogen in comet 67P/Churyumov-Gerasimenko indicates a low formation temperature,” by M. Rubin et al is published in the 20 March issue of the journal Science. 10.1126/science.aaa6100

ROSINA is the Rosetta Orbiter Spectrometer for Ion and Neutral Analysis
instrument and comprises two mass spectrometers: the Double Focusing
Mass Spectrometer (DFMS) and the Reflectron Time of Flight mass
spectrometer (RTOF) – and the COmetary Pressure Sensor (COPS). The
measurements reported here were conducted with DFMS. The ROSINA team is
led by Kathrin Altwegg of the University of Bern, Switzerland.

An average ratio of N2/CO = (5.70 +/- 0.66) x 10–3 was determined for the period 17–23 October 2014. The minimum and maximum values measured were 1.7 x 10–3 and 1.6 x 10–2,
respectively. Because the amount and composition of the gases change
with comet rotation and position of the spacecraft with respect to the
comet’s surface, an average value is used.

The 14N/15N ratio for the N2 in Comet
67P/Churyumov–Gerasimenko is assumed to be 441, the value for the
protosolar nebula as measured from Jupiter and the solar wind, while the
corresponding value for nitrogen in hydrogen cyanide and ammonia is
130, as measured at other comets. The value for the Earth’s nitrogen is
272.

More about Rosseta

Rosetta is an ESA mission with contributions from its Member States and
NASA. Rosetta’s Philae lander was provided by a consortium led by DLR,
MPS, CNES and ASI. Rosetta is the first mission in history to rendezvous
with a comet. It is escorting the comet as they orbit the Sun together.
Philae landed on the comet on 12 November 2014. Comets are time
capsules containing primitive material left over from the epoch when the
Sun and its planets formed. By studying the gas, dust and structure of
the nucleus and organic materials associated with the comet, via both
remote and in situ observations, the Rosetta mission should become the key to unlocking the history and evolution of our Solar System.